Conventionally, a metal foam has been known as a typical cellular light structure. This metal foam is manufactured by producing bubbles inside a metal of liquid or semi-solid state (Closed cell), or by casting the metal into a mold made of a foaming resin (Open cell). However, these metal foams have relatively inferior mechanical properties such as strength and rigidity. In addition, due to their high manufacturing cost, they have not been used widely in practice, except for a special purpose such as in airspace or aviation industries.
As a substitute material for the above mentioned metal foams, open cell-type light structures having periodic truss cells have been developed. This open cell-type light structure is designed so as to have an optimum strength and rigidity through precision mathematical and mechanical analysis, and therefore it has good mechanical properties. A typical truss structure is exemplified by the Octet truss where regular tetrahedrons and octahedrons are combined (See R. Buckminster Fuller, 1961, U.S. Pat. No. 2,986,241). Each element of the truss forms an equilateral triangle and thus it is advantageous in terms of strength and rigidity. Recently, as a modification of the Octet truss, the Kagome truss has been reported (See S. Hyun, A. M. Karlsson, S. Torquato, A. G. Evans, 2003. Int. J. of Solids and Structures, Vol. 40, pp. 6989-6998).
Referring to FIGS. 1a to 1b, the two-dimensional Octet truss 101 and the two-dimensional Kagome truss 102 are compared, that is, the unit cell 102a of the Kagome truss 102 has an equilateral triangle and a regular hexagon mixed in each face, dissimilar to the unit cell 101a of the Octet truss 101. FIGS. 2a-2c and 3a-3c show a single layer of the three-dimensional Octet truss 201 and the three-dimensional Kagome truss 202, respectively. Comparing the unit cell 201a of the three-dimensional Octet truss 201 with the unit cell 202a of the three-dimensional Kagome truss 202, one significant feature of the 3D Kagome truss 202 is that it has isotropic mechanical properties. Therefore, the structural materials or other materials based on the Kagome truss have a uniform mechanical and electrical property regardless of its orientation.
On the other hand, several processes have been used for manufacturing a cellular light structure of truss-type. First, a truss structure is formed of a resin and a metal is cast using the truss structure as a mold (See S. Chiras, D. R. Mumm, N. Wicks, A. G Evans, J. W. Hutchinson, K. Dharmasena, H. N. G. Wadley, S. Fichter, 2002, International Journal of Solids and Structures, Vol. 39, pp. 4093-4115). Second, a metallic net is formed by making periodic holes in a thin metal plate, a truss core is formed by crimping the metallic net, and face sheets are bent to the upper and lower portion thereof (See D. J. Sypeck and H. N. G. Wadley, 2002, Advanced Engineering Materials, Vol. 4, pp. 759-764). Here, in the case where a multi-layered structure having more than one layer is fabricated, another crimped-truss core is placed above the upper face sheet and another upper face sheet is positioned again above second core. In a third method, a wire-net is first woven using two orientational-wires perpendicular to each other, and then the wire-nets are laminated and bonded (See D. J. Sypeck and H. G. N. Wadley, 2001, J. Mater. Res., Vol. 16, pp. 890-897).
The manufacturing procedures of the first method are complicated, which leads to an increased manufacturing cost. Only metals having a good castability can be used and consequently it has limited applications. The resultant material tends to have casting defects and deficient mechanical properties. In the second method, the process making periodic holes in thin metal plates leads to loss of materials. Moreover, even though there is no specific problem in manufacturing a sandwiched plate material having a single-layered truss, the truss cores and face sheets must be laminated and bonded repeatedly so as to manufacture a multi-layered structure, thereby producing many bonding points which results in disadvantages of bonding cost and strength.
On the other hand, in case of the third method, the formed truss has no ideal regular tetrahedron or pyramid shape and thus has an inferior mechanical strength. Similar to the second method, lamination and bonding are involved to manufacture a multi-layered structure and therefore disadvantageous with respect to bonding cost and strength.
FIG. 4 shows a light structure manufactured by the third method, which is formed by laminating wire-nets. This method is known to be able to reduce the manufacturing cost, but wires of two orientations are woven like fabrics, and therefore it cannot provide an ideal truss structure having an optimum mechanical and electrical property as in the above-described three-dimensional Octet truss 201 or three-dimensional Kagome truss 202. Accordingly, it embraces disadvantages in terms of cost and strength, due to lots of portions to be bonded.
By the way, a common fiber reinforced composite material is manufactured in the form of a thin two-dimensional layer, which is laminated when a thick material is required. Due to a de-lamination phenomenon between the layers, however, its strength tends to be decreased. Therefore, first the fiber is woven in a three-dimensional structure, and then a matrix such as resin, metal, or the like is combined with the structure. FIGS. 5a-5b are perspective views of the woven fiber in this three-dimensional fiber-reinforced composite material. Instead of fibers, a material such as a metallic wire having a high stiffness can be woven into a three-dimensional cellular light structure as shown in FIGS. 5a-5b. However, it also does not have the above-described ideal Octet or Kagome truss structure so that it has a decreased mechanical strength and anisotropic material properties. Consequently, the composite material using the three-dimensional woven-fiber comes to have an inferior mechanical properties.